The field of the invention is the field of Radio Frequency (RF) transponders (RF Tags) which receive RF electromagnetic radiation from a base station and send information to the base station by modulating the load of an RF antenna.
RF Tags can be used in a multiplicity of ways for locating and identifying accompanying objects, items, animals, and people, whether these objects, items, animals, and people are stationary or mobile, and transmitting information about the state of the of the objects, items, animals, and people. It has been known since the early 60""s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components on a transponder could be powered by radio frequency (RF) power sent by a xe2x80x9cbase stationxe2x80x9d at a carrier frequency and received by an antenna on the tag. The signal picked up by the tag antenna induces an alternating current in the antenna which can be rectified by an RF diode and the rectified current can be used for a power supply for the electronic components. The tag antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the tag antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the tag antenna. This change in the radiated power from the tag antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. In the cited patent, the antenna current also oscillates at a harmonic of the carrier frequency because the diode current contains a doubled frequency component, and this frequency can be picked up and sorted out from the carrier frequency much more easily than if it were merely reflected. Since this type of tag carries no power supply of its own, it is called a xe2x80x9cpassivexe2x80x9d tag to distinguish it from an active tag containing a battery. The battery supplies energy to run the active tag electronics, but not to broadcast the information from the tag antenna. An active tag also changes the loading on the tag antenna for the purpose of transmitting information to the base station.
The xe2x80x9crebroadcastxe2x80x9d or xe2x80x9creflectionxe2x80x9d of the incoming RF energy at the carrier frequency is conventionally called xe2x80x9cback scatteringxe2x80x9d, even though the tag broadcasts the energy in a pattern determined solely by the tag antenna and most of the energy may not be directed xe2x80x9cbackxe2x80x9d to the transmitting antenna.
In the 70""s, suggestions to use tags with logic and read/write memories were made. In this way, the tag could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal. The antenna load was changed by use of a transistor. A transistor switch also changed the loading of the transponder in U.S. Pat. No. 4,786,907 by A. Koelle.
Prior art tags have used electronic logic and memory circuits and receiver circuits and modulator circuits for receiving information from the base station and for sending information from the tag to the base station.
The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such tags. Presently available research and development technology will also allow new function and different products in communications technology.
A fundamental problem with RF tags is that the cost of the tag must be reduced to a level small compared to the cost of the product to which the tag is attached, which would then allow many more tags to be used and so that high volume production can cut the costs even further. The cost of the tags is the cost of the semiconductor chip, the antenna, the substrate supporting the antenna and chip, and the attachment cost. The present invention lowers the cost and increases the function of the antenna attached to the substrate.
Related U.S. patents assigned to the assignee of the present invention include: U.S. Pat. No. 5,866,044, which is a continuation of application Ser. No. 08/2,339,609, filed Nov. 15, 1994 (now abandoned). Other U.S. patents include: U.S. Pat. Nos. 5,521,601; 5,528,222; 5,538,803; 5,550,547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106; 5,682,143; 5,729,201; 5,729,697; 5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; 5,821,859; 5,828,318; 5,831,532; 5,850,181; 5,874,902; 5,889,489; 5,909,176; and 5,912,632. U.S. patent applications assigned to the assignee of the present invention include: application Ser. No. 099/119,569, filed Jul. 20, 1998, which is a division of Ser. No. 08/734,492 (now U.S. Pat. No. 5,866,044. The above identified U.S. patents and U.S. patent applications are hereby incorporated by reference.
It is an object of the invention to produce an RF transponder comprising components which can be made at low cost. It is a further object of the invention to provide an RF transponder comprising a flexible antenna and substrate. It is a further object of the invention to provide an RF transponder which can be made by screen printing techniques. It is a further object of the invention to provide an RF transponder having enhanced environmental resistance. It is a further object of the invention to provide an RF transponder in a thin package. It is a further object of the invention to provide an RF transponder antenna which may easily be electrically connected to a semiconductor chip.
The present invention is an RF antenna made from a composite material, wherein the composite material preferably comprises electrically conducting particles in a matrix, and wherein the electrically conducting particles have such a high density that the electrical conductivity of the composite material is large enough for the antenna to receive RF signals sufficient to activate the RF tag.